I'm working on this exact thing right now, it's been a bit of a frustrating experience really. It seems like you have to jump through a bunch of hoops to make it work, and I've still not got it working quite how I'd like it.
What I'm trying to implement is using a category page to display images based on specific categories / child categories and then have them be viewable by a JS lightbox method. What I find clunky is that you need to attach all images to posts first before you can categorise them, this seems like an unnecessary step when it seems like there's no reason why an image couldn't have categories linked to it just like any other post in the wp_posts table - images are technically attachment post_type after all.
Being able to just upload images and categorise them at the time of upload without having to place them in a specific page seems much more sensible to me. Or am I missing some kind of benefit with the current method here?
The other issue I'm encountering in WP3.01 is that when I place an image in a post and then delete it that it is still remaining as an attachment even though I've removed it. Seems buggy to me but maybe I'm doing something wrong there too?